Although you can never really "ensure" it since seat assignments aren't guaranteed. As an elite member, you're much more likely to keep you chosen seat than a GM, but I've been bumped out of my chosen seat into E- even as a 2P.

Personally, I think airlines should offer guaranteed seat assignments, save for equipment changes. The compensation for losing your guaranteed seat wouldn't necessarily be much, but at least a) there would be some acknowledgment that you're being inconvenienced; and b) it would recognize that seat assignments are very important to some people (such as PTravel) and that the airline should try to minimize the disruption if they do have to reseat you. No more E+ window --> E- middle swaps without so much as an apology.